1. Field of the Invention
The present invention relates to a process for forming an oxide superconductor thin film and a laminate of oxide superconductor thin films that can be used for electronic elements.
2. Description of the Prior Art
As an oxide superconductor, there has heretofore been known a copper oxide superconductor (generally called 123-type copper superconductor) having the formula YBa.sub.2 Cu.sub.3 O.sub.7-x and an atomic ratio Y:Ba:Cu of substantially 1:2:3. This oxide superconductor has a layered crystal structure and exhibits strong anisotropic electric conduction property, and it has been desired to develop technology for uniformalizing axis orientations of crystals.
In preparing the junction, however, when the crystal axis orientations of an oxide superconductor thin film are all uniformalized, there no longer exists any interface, and neither a two-terminal element nor a three-terminal element can be realized with the thin film. To prepare a two-terminal element or a three-terminal element, therefore, a thin film of other material had to be laminated. In this lamination, however, the electric conduction in the interface of the laminate is subject to undergo a large change due to diffusion of atoms of different kinds, making it difficult to prepare ideal two-terminal element or three-terminal element.
It can therefore be contrived to prepare a two-terminal element or a three-terminal element in which the interface of the laminated layers is not changed by the diffusion of atoms of different kinds by utilizing strong anisotropy of the 123-type copper oxide superconductor and by laminating 123-type copper oxide superconductor thin films having dissimilar axis orientation as shown in FIG. 1.
The laminated layer structure of 123-type copper oxide superconductor thin films having dissimilar axis orientations has already been proposed in a "Report on the Study and Development of Large Current High Magnetic Field Superconductor Materials", International Superconductor Industries Technology Research Center Foundation, pp. 331-333. However, neither the conditions of lattice constants for laminating oxide superconductor thin films nor a suitable method of forming films has yet been known, and the above-mentioned two-terminal element or the three-terminal element has not been produced.
The laminated structure of 123-type copper oxide superconductor thin films having dissimilar axis orientations is also required for integrating the Josephson element. Among the 123-type copper oxide superconductor Josephson elements that have now been studied, the structure having a high degree of integration is a laminated structure. Among the elements of the laminated structure, the one that has a good Josephson junction is obtained by laminating a-axis oriented single crystalline thin films as disclosed in "Current State of High Temperature Superconductive Electronic devices" by Susumu Takada, Applied Physics, Vol. 62, No. 5, 1993, pp. 443-454. When the a-axis oriented single crystalline thin film is used as the Josephson element, however, a wiring must be formed by using the a-axis oriented single crystalline thin film having a low critical current Ic since the existing technology is capable of laminating films having the same axis orientation only. Accordingly, the wiring becomes too thick and the degree of integration cannot be so increased. In order to solve this problem, it becomes necessary to establish technology for laminating the a-axis oriented single crystalline thin film upon the c-axis oriented single crystalline thin film, in order to laminate an element of the a-axis oriented film that can be highly densely integrated on a wiring of the c-axis oriented single crystalline thin film that has a high critical current Ic and can be highly densely integrated.
It is not allowed to laminate 123-type copper oxide superconductor thin films having dissimilar crystal axis orientations because of the reasons described below.
What determines the crystal axis orientation of the thin film is the crystalline structure in the surface of the substrate. In order to laminate a oxide superconductor thin film YBa.sub.2 Cu.sub.3 O.sub.7-x having dissimilar crystal axis orientations, the substrate must be a single crystal. When the surface of the substrate is polycrystalline, it is allowed to obtain the c-axis oriented polycrystalline film only since the crystals of the 123-type copper oxide superconductor thin film rapidly grow along the a-b plane. Since this thin film is polycrystalline, the 123-type copper oxide superconductor thin film laminated thereon is oriented in the c-axis direction only. That is, the laminated films of the same axis orientation only are formed.
When the crystalline structure in the surface of the substrate is of a single crystal, a single crystal thin film (lower layer) of the 123-type copper oxide superconductor can be formed thereon. In laminating the 123-type copper oxide superconductor thin film (upper layer) thereon, it becomes possible to form a single crystal thin film of the 123-type copper oxide superconductor having a dissimilar crystal axis orientation when the lattice constants are different between the upper layer and the lower layer at the time of forming films. In practice, however, neither lattice constants of the upper layer and the lower layer nor the method of forming films for realizing the lattice constants have been known. Therefore, the upper layer is formed having the same crystal axis orientation as the lower layer, and it has not yet been succeeded in laminating a thin oxide film of a layered crystal structure having a dissimilar crystal axis orientation.